A hydraulic drive system equipped with a load sensing system for controlling the delivery flow rate of a hydraulic pump (main pump) such that the delivery pressure of the hydraulic pump becomes higher by a target differential pressure than the maximum load pressure of a plurality of actuators is widely used today as the hydraulic drive systems for construction machines such as hydraulic excavators.
A hydraulic drive system for a construction machine equipped with such a load sensing system is described in Patent Literature 1, in which a two-pump load sensing system including two hydraulic pumps (first and second hydraulic pumps) corresponding to first and second actuator groups is employed. In the two-pump load sensing system, the maximum displacement of one of the two hydraulic pumps (first hydraulic pump) is set larger than the maximum displacement of the other hydraulic pump (second hydraulic pump). The maximum displacement of the first hydraulic pump is set at a level enough for driving an actuator whose maximum demanded flow rate is the highest (assumed to be an arm cylinder). A specific actuator (assumed to be a boom cylinder) is driven by the delivery flow from the second hydraulic pump. Further, a confluence valve is arranged on the first hydraulic pump's side. Only when the demanded flow rate of the actuator whose maximum demanded flow rate is the highest (assumed to be the arm cylinder) is low, it is made possible to merge the delivery flow from the first hydraulic pump with the delivery flow from the second hydraulic pump via the confluence valve and supply the merged delivery flow to the specific actuator (assumed to be the boom cylinder) when the demanded flow rate of the specific actuator (assumed to be the boom cylinder) is high.
Patent Literature 2 describes a two-pump load sensing system in which a hydraulic pump of the split flow type having two delivery ports is employed instead of two hydraulic pumps. In this system, the delivery flow rates of first and second delivery ports can be controlled independently of each other based respectively on the maximum load pressure of a first actuator group and the maximum load pressure of a second actuator group. Also in this system, a separation/confluence selector valve (travel independent valve) is arranged between the delivery hydraulic lines of the two delivery ports. In cases like performing the traveling only or using the dozer equipment while traveling, the separation/confluence selector valve is switched to a separation position and the delivery flows from the two delivery ports are supplied independently to the actuators. In cases of driving actuators not for the traveling or the dozer (e.g., boom cylinder, arm cylinder, etc.), the separation/confluence selector valve is switched to a confluence position so that the delivery flows from the two delivery ports can be merged together and supplied to the actuators.